Friday, January 13, 2012

The 40 FPM Quandry

We, as well as some of our competitors, have had engineers call and ask if we can guarantee that our air distribution system can guarantee to achieve less than 40 fpm at all points in the occupied zone, in order to meet LEED thermal comfort requirements. The fact is that no overhead system can guarantee meeting this requirement. The requirement comes from a statement in ASHRAE Standard 55 that the Graphical Method, "Figure 5.2.1.1 specifies the comfort zone for environments that meet the above criteria and where the air speeds are not greater than 0.20 m/s (40 ft/min)." The graphical method is the most used method in the standard, as it is the easiest.

In another section of the 55 Standard; however, it states "the designer shall decide the proper averaging for air speed for use in the Graphical Method (5.2.3.1)". One method (in fact the only method) of estimating room air speeds is to use the ADPI methodology described in the ASHRAE Fundamentals Handbook, chapter 20. If the ADPI is calculated to be at least 80%, the average room air speed can be assured to be less than 40fpm. In fact, it is usually no higher than 30 fpm. This relationship will be included in an addition to the Handbook in the near future.

In addition, an ADPI >80% will also assure that there is less than the maximum 5.4F vertical temperature stratification in the occupied zone, another Standard 55 requirement. Thus, if the engineer chooses to use ADPI as the method of estimating air speed, he will be allowed to use the graphical method of determining design temperatures and assuring compliance to the stratification requirement as well.

Krueger has the best, and easiest, ADPI selection methods in the industry, including ADPI selection graphs in the catalog and an easy to use ADPI calculator and printable graph output in K-Select. We also provide the only available Standard 55 Graphical Method computer program as a free download on our website. (http://www.krueger-hvac.com/tools/comfort.asp)

Authored by: Dan Int-Hout, Chief Engineer Krueger